Background: Systemic glucocorticoids are a major therapy for the management of allergic inflammation and asthma; however, information about their effects in vivo are limited. Objective: This study was performed to examine the effects of prednisone on inflammatory mediators, cytokines, and cellular responses in the model of segmental allergen challenge (SAC) of allergic asthmatic subjects. Methods: The effects of a 3-day pretreatment with oral prednisone (30 mg twice daily) on the physiologic and inflammatory responses to SAC were studied in 10 allergic asthmatic subjects in a double-blind, placebo-controlled, crossover protocol. Results: Prednisone improved baseline FEV₁ by 10% and modestly inhibited the SAC-induced fall in FEV₁ at 30 minutes and at 6 to 8 hours. Five minutes after challenge, levels of histamine, PGD₂, 9⍺,11β-PGF₂, and thromboxane B₂ increased in bronchoalveolar lavage fluid (median increase, 5- to 14-fold); prednisone did not inhibit these responses. Prednisone inhibited (median decrease, 66%-97%) the total influx of inflammatory cells, specifically eosinophils, basophils, and some subsets of T lymphocytes (CD4, CD45RA, and CD45RO cells) assessed 19 hours after SAC, but it did not inhibit the influx of neutrophils. Increases in soluble E-selectin, kinins, and albumin were also inhibited by the glucocorticoid (median decrease, 36%-74%). Prednisone treatment inhibited the appearance of mRNA, protein, or both for T_H2 cytokines (IL-4 and IL-5), as well as for IL-2 and transforming growth factor ⍺, but did not inhibit increases of immunoreactive GM-CSF in bronchoalveolar lavage fluid. Conclusion: These studies indicate that prednisone suppresses multiple components of allergic airway inflammation, including cell recruitment, adhesion molecule expression or release, airway permeability, and production of cytokines potentially involved in airway immunity or remodeling. (J Allergy Clin Immunol 2001;108:29-38.)